We have previously shown that p53 mutations are associated withcisplatin resistance in ovarian carcinoma IGROV-1/Pt 1 cells.The relationship between p53 status and the development of resistancehas not been completely elucidated; in particular, the biologicalmechanisms behind the acquired drug-resistant p53-mutant phenotypewere not clearly explained. Thus, in this study, we investigatedwhether the p53 mutations found in IGROV-1/Pt 1 cells (270 and282 codons) resulted from selection, under the selective pressureof the cytotoxic treatment, of a spontaneously mutant cell populationpreexistent in the cisplatin-sensitive parental cell line (IGROV-1)or were induced by drug (genotoxic) treatment. For this purpose,an allele-specific PCR approach was used. Primers carrying thedesired mutations (TA codon 270, CT codon 282) in the 3' terminus,and the corresponding wild-type primers were used to amplifygenomic DNA from the original IGROV-1 cell line used to selectthe mutant IGROV-1/Pt 1. To increase sensitivity, we hybridizedblots of the PCRs with the radiolabeled PCR fragment from IGROV-1/Pt1. Amplification was obtained for IGROV-1 DNA with the mutatedallele-specific primers, indicating the preexistence of a mutatedpopulation in the IGROV-1 cell line. Titration experiments suggestedthat the frequency of the mutated alleles was <0.1%. Single-strandconformation polymorphism and allele-specific PCR analysis ofthe IGROV-1/Pt 0.1 cells, which are less resistant to cisplatinthan IGROV-1/Pt 1 cells and which carry both mutant and wild-typep53 alleles with a wild-type predominance, suggested a progressiveselection of the mutant population by cisplatin treatment. Thisis the first observation that indicates that a subpopulationof p53 mutant cells can occasionally be selected by cisplatintreatment. Thus, considering the susceptibility to spontaneousmutations of the p53 gene in advanced ovarian carcinoma, theselection process resulting in emergence of p53 mutant tumorsis a possible origin of resistance of ovarian carcinoma to DNA-damagingagents. The survival advantage of p53 mutant cells in the presenceof genotoxic agents could be related to a loss of susceptibilityto p53-dependent apoptosis and to defects in checkpoints pathways,resulting in genomic instability.